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how much different is the total rolling radius (is anyone counting these)...

16in x 55 mm sidewall = 16 * 2.54 + 5.5 + 5.5 = 51.14 Diameter

17in x 40 mm sidewall = 17 * 2.54 + 4 + 4 = 51.18 Diameter

18in x 30mm sidewall = 18 x 2.54 + 7 = 52.72 Diameter

circumference = pi x D... work out the % diff by old circ. / new circ. These should all give you close enough to stock that your speedo will hardly be affected.

does this all sound like high school maths all over again?

and a smaller circumference will give you more acceleration, larger more top speed (if you could do it), same as a diff ratio will.

Keep the same rolling radius... easier all round, no issues with scrubbing, no issues with speedo.

BTW - my speedo is 6% out - as in it reads 94% of actual speed... with stock rims and rubber.

--ebola

how much different is the total rolling radius (is anyone counting these)...

16in x 55 mm sidewall = 16 * 2.54 + 5.5 + 5.5 = 51.14 Diameter

17in x 40 mm sidewall = 17 * 2.54 + 4 + 4 = 51.18 Diameter

18in x 30mm sidewall = 18 x 2.54 + 7 = 52.72 Diameter

circumference = pi x D... work out the % diff by old circ. / new circ. These should all give you close enough to stock that your speedo will hardly be affected.

does this all sound like high school maths all over again?

and a smaller circumference will give you more acceleration, larger more top speed (if you could do it), same as a diff ratio will.

Keep the same rolling radius... easier all round, no issues with scrubbing, no issues with speedo.

BTW - my speedo is 6% out - as in it reads 94% of actual speed... with stock rims and rubber.

--ebola

Too much maths and the wrong maths.You don't need to know the circumference as it's directly proportional to the diameter.Also it's not 55mm sidewalls.It's 55% of 205mm which is 112.75mm.Overall diameter of 205/55/16 is 63.19cm.

I worked out the differences for my own car. 235/40/18=64.52cm diameter.

Difference is 2%,so instead of indicating 100k's it'd show 98k's.

Stuff all.Not worth worrying about.

Oh yeah,235/45/17 is slightly less,about 1.8%.

Edited by wun hung lo

Or if you do want to measure the circumference you can try this method.

rpm/gear ratio/diff ratio x wheel circumference in kms x 60 = kph

example is my r 32 GTR

at 7000 in 4th gear ( 1:1 ) with 4.11 diff's and 18" 245/40/18 (2080mm )

7000/1.1/4.11 x .00208kms x 60 = 212Kph while the speedo was happily sitting on 195kph which is a brand new Nismo 320kph jobbie

Edited by tacker
Too much maths and the wrong maths.You don't need to know the circumference as it's directly proportional to the diameter.Also it's not 55mm sidewalls.It's 55% of 205mm which is 112.75mm.Overall diameter of 205/55/16 is 63.19cm.

I worked out the differences for my own car. 235/40/18=64.52cm diameter.

Difference is 2%,so instead of indicating 100k's it'd show 98k's.

Stuff all.Not worth worrying about.

Oh yeah,235/45/17 is slightly less,about 1.8%.

:O:):)

and oops - pi x D, pi is constant.

and 55% means that my 205/55R16's are smaller than 235/55R16 wheels.... !?!?!?

then again, you are right that its *not* 55mm, so, umm, yeah.

205/55 is a fair bit smaller than 235/55.

The proper correct size which is only a TINY bit (</=1%) bigger than stock is 225/45R17, 255/40R17, 225/40R18, 255/35R18

Sometimes these sizes are more expensive than the usual 235 or 265 but then again, sometimes they are cheaper.

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